This invention involves the preparation of anionic clays and the preparation of Mg--Al solid solutions by heat-treatment of the anionic clay. Anionic clays have a crystal structure which consists of positively charged layers built up of specific combinations of metal hydroxides between which there are anions and water molecules. Hydrotalcite is an example of a naturally occurring anionic clay, in which carbonate is the predominant anion present. Meixnerite is an anionic clay wherein hydroxyl is the predominant anion present.
In hydrotalcite-like anionic clays the brucite-like main layers are built up of octahedra alternating with interlayers in which water molecules and anions, more particularly carbonate ions, are distributed. The interlayers may contain anions such as NO.sub.3.sup.-, OH, Cl.sup.-, Br.sup.-, I.sup.-, SO.sub.4.sup.2-, SiO.sub.3.sup.2-, CrO.sub.4.sup.2-, BO.sub.3.sup.2-, MnO.sub.4.sup.-, HGaO.sub.3.sup.2-, HVO.sub.4.sup.2-, ClO.sub.4.sup.-, BO.sub.3.sup.2-, pillaring anions such as V.sub.10 O.sub.26.sup.-6 and Mo.sub.7 O.sub.24.sup.6-. monocarboxylates such as acetate, dicarboxylates such as oxalate, alkyl sulphonates such as laurylsulphonate.
It should be noted that a variety of terms are used to describe the material which is referred to in this patent as an anionic clay. Hydrotalcite-like and layered double hydroxide are interchangeably used by those skilled in the art. In this patent application we refer to the materials as anionic clays, comprising within that term hydrotalcite-like and layered double hydroxide materials.
The preparation of anionic clays has been described in many prior art publications.
Recently, two major reviews of anionic clay chemistry were published in which the synthesis methods available for anionic clay synthesis have been summarized, F. Cavani et al "Hydrotalcite-type anionic clays: Preparation, Properties and Applications," Catalysis Today", 11 (1991) Elsevier Science Publishers B. V. Amsterdam.
J P Besse and others "Anionic clays:trends in pillary chemistry, its synthesis and microporous solids" (1992), 2, 108, editors: M. I. Occelli, H. E. Robson, Van Nostrand Reinhold, N. Y.
In these reviews the authors state that a characteristic of anionic clays is that mild calcination at 500.degree. C. results in the formation of a disordered MgO-like product. Said disordered MgO-like product is distinguishable from spinel (which results upon severe calcination) and from anionic clays. In this patent application we refer to said disordered MgO-like materials as Mg--Al solid solutions. Furthermore, these Mg--Al solid solutions contain a well-known memory effect whereby the exposure to water of such calcined materials results in the reformation of the anionic clay structure
For work on anionic clays, reference is given to the following articles:
Helv. Chim. Acta, 25, 106-137 and 555-569 (1942) PA0 J. Am. Ceram. Soc., 42, no. 3, 121 (1959) PA0 Chemistry Letters (Japan), 843 (1973) PA0 Clays and Clay Minerals, 23, 369 (1975) PA0 Clays and Clay Minerals, 28, 50 (1980) PA0 Clays and Clay Minerals, 34, 507 (1996) PA0 Materials Chemistry and Physics, 14, 569 (1986).
In addition there is an extensive amount of patent literature on the use of anionic clays and processes for their preparation
European Patent Application 0 536 879 describes a method for introducing pH-dependent anions into the clay. The clay is prepared by the addition of a solution of Al(NO.sub.3).sub.3 and Mg(NO.sub.3).sub.2 to a basic solution containing borate anions. The product is then filtered, washed repeatedly with water, and dried overnight. Additionally mixtures of Zn/Mg are used.
In U.S. Pat. No. 3,796,792 by Miyata entitled "Composite Metal Hydroxides" a range of materials is prepared into which an extensive range of cations is incorporated, including Sc, La, Th, In, etc. In the examples given solutions of the divalent and trivalent cations are prepared and mixed with base to cause co-precipitation. The resulting products are filtered, washed with water, and dried at 80.degree. C. Example 1 refers to Mg and Al and Example 2 to Mg and Bi. Other examples are given, and in each case soluble salts are used to make solutions prior to precipitation of the anionic clay at high pH.
In U.S. Pat. No. 3,879,523 by Miyata entitled "Composite Metal Hydroxides" also a large number of preparation examples is outlined. The underlying chemistry, however, is again based on the co-precipitation of soluble salts followed by washing and drying. It is important to emphasize that washing is a necessary part of such preparations, because to create a basic environment for co-precipitation of the metal ions a basic solution is needed and this is provided by NaOH/Na.sub.2 CO.sub.3 solutions. Residual sodium, for example, can have a significant deleterious effect on the subsequent performance of the product as a catalyst or oxide support.
In U.S. Pat. No. 3,879,525 (Miyata) very similar procedures are again described.
In U.S. Pat. No. 4,351,814 to Miyata et al. a method for making fibrous hydrotalcite is described. Such materials differ in structure from the normal plate-like morphology. The synthesis again involves soluble salts. For example, an aqueous solution of a mixture of MgCl.sub.2 and CaCl.sub.2 is prepared and suitably aged. From this a needle-like product Mg.sub.2 (OH).sub.3 Cl.4H.sub.2 O precipitates. A separate solution of sodium aluminate is then reacted in an autoclave with the solid Mg.sub.2 (OH).sub.3 Cl.4H.sub.2 O and the product is again filtered, washed with water, and dried.
In U.S. Pat. No. 4,458,026 to Reichle, in which heat-treated anionic clays are described as catalysts for aldol condensation reactions, again use is made of magnesium and aluminum nitrate salt solutions. Such solutions being added to a second solution of NAOH and Na.sub.2 CO.sub.3. After precipitation the slurry is filtered and washed twice with distilled water before drying at 125.degree. C.
In U.S. Pat. No. 4,656,156 to Misra the preparation of a novel absorbent based on mixing activated alumina and hydrotalcite is described. The hydrotalcite is made by reacting activated MgO (prepared by activating a magnesium compound such as magnesium carbonate or magnesium hydroxide) with aqueous solutions containing aluminate, carbonate and hydroxyl ions. As an example the solution is made from NAOH, Na.sub.2 CO.sub.3 and Al.sub.2 O.sub.3. In particular, the synthesis involves the use of industrial Bayer liquor as the source of Al. The resulting products are washed and filtered before drying at 105.degree. C.
In U.S. Pat. No. 4,904,457 to Misra a method is described for producing hydrotalcites in high yield by reacting activated magnesia with an aqueous solution containing aluminate, carbonate, and hydroxyl ions.
The methodology is repeated in U.S. Pat. No. 4,656,156.
In U.S. Pat. No. 5,507,980 to Kelkar et at al. a process is described for making novel catalysts, catalyst supports, and absorbers comprising synthetic hydrotalcite-like binders. The synthesis of the typical sheet hydrotalcite involves reacting pseudo-boehmite to which acetic acid has been added to peptize the pseudo-boehmite. This is then mixed with magnesia. More importantly, the patent summary states clearly that the invention uses mono carboxylic organic acids such as formic, propionic and isobutyric acid. In this patent the conventional approaches to preparing hydrotalcite are presented.
In U.S. Pat. No. 5,439,861 a process is disclosed for preparing a catalysts for synthesis gas production based on hydrotalcite. The method of preparation is again based, on the co-precipitation of soluble salts by mixing with base, for example, by the addition of a solution of RhCl.sub.3, Mg(NO.sub.3).sub.2 and Al(NO.sub.3).sub.3 to a solution of Na.sub.2 CO.sub.3 and NaOH.
Also in U.S. Pat. No. 5,399,537 to Bhattacharyya in the preparation of nickel-containing catalysts based on hydrotalcite use is made of the co-precipitation of soluble magnesium and aluminum salts.
In U.S. Pat. No. 5,591,418 to Bhattacharyya a catalyst for removing sulphur oxides or nitrogen oxides from a gaseous mixture is made by calcining an anionic clay, said anionic clay having been prepared by co-precipitation of a solution of Mg(NO.sub.3).sub.2, Al(NO.sub.3).sub.3 and Ce(NO.sub.3).sub.3. The product again is filtered and repeatedly washed with de-ionized water.
In U.S. Pat. No. 5,114,898/WO 9110505 Pinnavaia et al. describe layered double hydroxide sorbents for the removal of sulphur oxide(s) from flue gases, which layered double hydroxide is prepared by reacting a solution of Al and Mg nitrates or chlorides with a solution of NAOH and Na.sub.2 CO.sub.3. In U.S. Pat. No. 5,079,203/WO 9118670 layered double hydroxides intercalated with polyoxo anions are described, with the parent clay being made by co-precipitation techniques.
In U.S. Pat. No. 5,578,286 in the name of Alcoa a process for the preparation of meixnerite is described. The meixnerite may be contacted with a dicarboxylate or polycarboxylate anion to form a hydrotalcite-like material. In comparative examples 1-3 hydromagnesite is contacted with aluminum trihydrate in a CO.sub.2 atmosphere, greater than 30 atmospheres. No hydrotalcite was obtained in these examples.
In U.S. Pat. No. 5,514,316 a method for the preparation of meixnerite is described using magnesium oxide and transition alumina. For comparative purposes aluminum trihydrate was used in combination with magnesium oxide. It was indicated that this method did not work as well as with transition alumina.
U.S. Pat. No. 4,454,244 and U.S. Pat. No. 4,843,168 describe the use of pillaring anions in anionic clays.
In U.S. Pat. No. 4,946,581 and U.S. Pat. No. 4,952,382 to van Broekhoven co-precipitation of soluble salts such as Mg(NO.sub.3).sub.2 and Al(NO.sub.3).sub.3 with, and without the incorporation of rare earth salts was used for the preparation of anionic clays as catalyst components and additives. A variety of anions and di- and tri-valent cations are described.
As indicated in the description of the prior art given-above, there are many applications of anionic clays.
These include but are not restricted to: catalysts, adsorbents, drilling muds, catalyst supports and carriers, extenders and applications in the medical field. In particular van Broekhoven has described their use in SO.sub.x abatement chemistry.
Because of this wide variety of large-scale commercial applications for these materials, new processes utilizing alternative inexpensive raw materials and which can be carried out in continuous mode are needed to provide a more cost-effective and environmentally compatible processes for making anionic clays. In particular, from the prior art described above one can conclude that the preparation process can be improved in the following ways: the use of cheaper sources of reactants, processes for easier handling of the reactants, so that there is no need for washing or filtration, eliminating the filtration problems associated with these fine-particled materials, the avoidance of alkali metals (which can be particularly disadvantageous for certain catalytic applications): In prior art preparations organic acids were used to peptize alumina. The use of organic acids is expensive and introduces an additional step in the synthesis process and is therefore not cost-effective. Further, in drying or calcining the anionic clay prepared by prior art processes gaseous emissions of nitrogen oxides, halogens, sulphur oxides, etc. are encountered which cause environmental pollution problems.